Theory of forms
The Theory of forms is a philosophical idea held by the famous Greek philosopher Plato. Plato believed that behind every single thing in our world there is a form, which is the true eternal essence of that thing. We can explain this view with an example. In the world there are many different horses: big horses and small horses, grey horses and white horses, old horses and young horses and so forth. According to Plato each horse is an imperfect copy of the horse 'form', the one true horse. In other words, they are all imitations of the perfect horse. Plato uses shadows as an analogy: Like a tree forms a shadow, so is the tree itself a shadow of the 'form', the one true tree. The Theory of forms applies not only to physical objects but also to abstract concepts such as beauty, anger, good and evil.
Platonic idealism also explains that it is impossible for us to perceive these forms through our senses, like our sense of hearing or sense of seeing. The only way we can truly understand a form is through the use of logic and mathematics. Through mathematics, we have discovered the form of the triangle: a polygon with 3 sides. However we will never be able to truly see with our eyes such a triangle. Even if we tried to draw it on a whiteboard with a ruler, it's lines will never be perfectly straight and two-dimensional.